SANAA, Yemen (AP) — Yemeni warplanes and troops backed by heavy artillery waged a four-front assault Tuesday against the strongholds of al-Qaida militants in the south, with U.S. troops for the first time helping direct the offensive from a nearby desert air base-turned-command center.

Yemeni military officials said dozens of U.S. troops were operating from al-Annad air base, about 65 kilometers (45 miles) from the main battle zones, coordinating assaults and airstrikes and providing information to Yemeni forces.

The officials said it was the most direct American involvement yet in the country’s expanding campaign against al-Qaida’s branch in Yemen, which has been blamed for directing a string of unsuccessful bomb plots on U.S. soil from its hideouts in the impoverished country at the tip of the Arabian Peninsula.

Most recently, this month it emerged that the CIA thwarted a plot to down a U.S.-bound airliner using a new, sophisticated explosive to be hidden in the bomber’s underwear. But the planned bomber was actually a double agent who turned the device over to the U.S. government.

The offensive is the most concerted yet aiming to uproot al-Qaida militants who since last year have held a swath of territory, including the provincial capital Zinjibar and several other towns, in the south of the country. One Yemeni military official said the country’s defense minister and an American general, whom he did not identify, were jointly overseeing the assault.

The Yemeni military officials, who are familiar with the workings of the army in the south, spoke on condition of anonymity to discuss the highly sensitive cooperation. The U.S. Embassy in Sanaa could not be reached for comment.

In a crescent-shaped assault on Zinjibar, Yemeni troops pushed into the center of the city, though they did not outright control it, one official said. Military helicopters flew over the city for the first time in an indication militants had lost their heavy weaponry capable of shooting down the helicopters, the official said.

The troops “can for the first time catch a glimpse of the torched government buildings” that al-Qaida’s fighters had hunkered down in during recent battles, the official said.

Al-Qaida militants seized Zinjibar, the capital of Abyan province on the Arabian Sea coast, last year while the country was mired in the political turmoil of the popular uprising against then-President Ali Abdullah Saleh. The militants also took control of several other nearby towns. Tens of thousands of residents fled the area.

Saleh, once a U.S. ally, finally stepped down in February under a Gulf-mediated, U.S.-backed deal.

For the past three months, the al-Qaida militants have carried out bloody attacks on Yemeni forces and raided weapons depots, capturing thousands of weapons, including assault rifles, machine-guns and even tanks, armored vehicles and rockets.

Yemen’s military has been largely ineffectual in uprooting the militants. The force is ill-equipped, poorly trained with weak intelligence capabilities and is riven with conflicted loyalties, since some commanders remain close to Saleh.

Saleh’s successor, Abed Rabbo Mansour Hadi, however, has vowed to make the fight against al-Qaida a priority. He moved commanders of army units, removed Saleh’s relatives in key security positions and tried to reach out to tribal leaders in the troubled south to form a strong front in the face of the militant group.

On Tuesday, the international leader of al-Qaida, Ayman al-Zawahri, released an audio recording online aimed at swaying public opinion against Hadi, calling him a U.S. agent and a traitor for having served as vice president during the “corrupt rule” of Saleh.

“Out went a (U.S.) agent and in came an agent,” al-Zawahri said. “How can Ali Abdullah Saleh be a criminal, murderer, thief, corrupt, traitor agent and Rabbo Mansour be the straighforward, honest, clean angel and the awaited savior?”

He warned the Yemenis against “U.S. plots” to manipulate Yemen in order to stack the situation in Washington’s favor.

Several Yemeni military officials told The Associated Press on Tuesday that unlike in previous, failed offensives against the Yemeni branch of al-Qaida, this time the United States was providing direct logistical support to the Yemeni forces.

Nearly 60 U.S. troops were at al-Annad base in Lahj province, neighboring Abyan, which has become a command center. “They brought their mobile houses and buildings for a long stay,” one official said. Another official said that along with coordinating the assault, U.S. personnel at the base were overseeing strikes by U.S. drone aircraft.

On Sunday, al-Qaida fighters attempted an attack on the northern gate of al-Annad air base, close to the troops’ living quarters, but were repelled. One Yemeni officer was killed in the attack, the officials said, and the Yemeni military later deployed heavy troops to protect the base.

The White House’s top counterterrorism adviser, John Brennan, met with Hadi on Sunday in the capital Sanaa, and the Yemeni leader briefed him on the army’s progress in the south, according to Hadi’s office. Defense Mohammed Nasser Ahmed described the operation as the “final decisive battle against al-Qaida.”

The Pentagon said a week ago that it had sent military trainers back to Yemen for “routine” counterterrorism cooperation with Yemeni security forces. A U.S. official said the troops are special operations forces, who work under more secretive arrangements than conventional U.S. troops and whose expertise includes training indigenous forces. The official spoke on condition of anonymity because he was not authorized to discuss the subject publicly.

Under Saleh, Washington had greatly expanded counterterrorism aid, at one point having between 100 and 150 trainers there. But the training program was suspended last year amid the revolt.

The U.S. also has a substantial naval presence near Yemen. U.S. Navy ships arrived in the area over the weekend on a routine rotation, carrying about 2,000 Marines aboard vessels including the amphibious assault ship Iwo Jima.

Al-Qaida’s branch in Yemen, known as al-Qaida in the Arabian Peninsula, was behind the failed Christmas 2009 attempt to bomb an American airliner as well as a foiled attempt the following year to mail package bombs to the U.S.

Aside from the assault on Zinjibar, warplanes were bombing al-Qaida hideouts in the nearby town of Jaar to the north. One raid hit a house, killing two militants. When residents went to inspect the site, a second raid mistakenly killed eight of the civilians and wounded 20, Yemeni military officials. Officials say that a total number of 17 al-Qaida militants and 18 army troops were killed over the past three days.

In a third front, Yemeni forces drove militants out of the town of Hurour, west of Zinjibar.

Abdu Dail, who fled Hurour with his family on Sunday, said most residents left after the military warned them about the upcoming offensive. On Sunday, airstrikes killed at least 30 militants.

Yemeni troops backed by armed civilian volunteers and airstrikes also assaulted militant positions at Youssef Mountain on the outskirts of the town of Lawder, where residents backed by the military drove out militants last year.

Abyan Governor Abyan Gamal al-Aqil told AP that civilian volunteers had seized several positions. Ali Aide, one of the citizesn-turned-fighters, said 14 militants, six fighters and two army troops were killed.

Yemeni military officials said uprooting militants from Zinjibar would deprive the group of its only major city, leaving them scattered in desert and mountain areas. It would also push militants back away from Aden, one of the most strategically important ports in Yemen on the Arabian Sea. Officials say that al-Qaida while controlling Zinjibar, has had its eyes on the province to the west, Aden.